Tackling Hurdles in Microwave-Assisted Close Vessel Digestion of Carbon-Rich Samples

Tackling Hurdles in Microwave-Assisted Close Vessel Digestion of Carbon-Rich Samples

Microwave-assisted sample digestions techniques excel in decomposing samples completely by raising the temperature of acid higher than the boiling point. The key to increasing the boiling point of acids is their entrapment in Teflon^® lined close digestion vessel. Carbon-rich samples such as vegetation, food, oil, wax, etc., upon reaction with acid, generate gases that increase the pressure build-up in the vessel. The pressure build-up is substantially more comparable to what would be otherwise with water at the same temperature. The pressure sustaining the capacity of digestion vessels then limits the amount of carbon-reach samples that can be digested. These samples invariably have a very low concentration of elements of interest. The typical sample quantity in the range of 100 to 200 mg does not provide an appropriate signal for spectroscopy instruments to estimate concentration with confidence. The analyst seems to have been trapped in the dilemma of whether to decreasing digestion temperatures or sample quantity is way out, as both approaches affect the analytical evaluations adversely.

By and large carbons rich sample have two categories of carbon compounds, aliphatic and aromatic. The aliphatic carbon can be attacked by nitric acid at or under the boiling point. If these samples are treated before treating it into a Microwave Digestion System can help reduce the pressure built-up by half at peak temperatures during microwave-assisted digestions. Effectively sample pre-treatment can be doubling the sample quantity that can be handled for specific vessel type. If the pre-treatment container is not the same as used later in the microwave digestion system, the situation will bring in the need to transfer the sample. The additional reagent will be required to transfer the sample introducing a possible loss of and dilution of the sample. Questron Technologies provides the digestion block that accommodated the Teflon^® liners from the Microwave Digestion System model QLABPro and QWAVE. The availability of such a digestion block eliminates the need for sample transfer after pre-treatment.

Pre-treating the samples with aromatic carbon as a major constituent of the sample does not reduce the built-up pressure while treating them in a microwave digestion system. In such a situation, two-stage digestion can come handy. In the first stage, the sample is treated at 160 °C and once the sample cools down below 100 °C, residual pressure is released. It is essential to release excess pressure in a controlled manner without opening the vessel to avoid loss of sample and acid condensate on the interior of the vessel cap. Questron eVHP vessel is ideally suited for this purpose. The PRV placed on each vessel top can bring down the pressure in the vessel to an atmospheric level through a controlled release of the gas produced as the by-product of digestion reaction. The pressure limit to which PRV can hold the pressure in the vessel is easily adjusted by simply turning of a knob.  PRV is set back to contain the high pressure after controlled release for the second stage of digestion.

In the second stage of the digestion vessel with an undigested portion of the sample is heated to 190 °C where pressure developed will be under the safe working limits of  Digestion Vials. Thus, with the help of the controlled release of reaction gas through PRV, it is possible to handle larger sample quantity.

The effective use of digestion blocks and smart vessel design for microwave-assisted digestion from Questron Technologies Corp. can tackle the hurdle of high-pressure during microwave-assisted digestion of carbon-rich samples.